Conventionally, a decolorable colorant capable of erasing colors, such as ink containing a leuco dye was used to print images on a paper in order to reuse papers on which images were printed by an image forming apparatus such as MFP (Multi-Function Peripheral). The paper printed with the decolorable colorant can be reused for printing since a chemical reaction occurs therein when heat or the like is applied thereto, thus decoloring the decolorable colorant.
However, when printing is performed using a colorant which is made colorless with heat or the like in order to make a paper reusable, it is difficult to maintain images for a longer period of time compared to a printing method that uses a general non-decolorable toner and ink and the like. For example, when a paper printed with a colorant which is made colorless with heat is left in a place such as an inside of a vehicle that is heated in the summer season, the image on the paper will be decolored, and accordingly, the information printed on the paper will disappear. In addition, when a user carelessly performs processes that erase images printed with a decolorable colorant with the aim to recycle a paper, the printed information will disappear.
Moreover, another problem is the difficulty to discriminate at a glance between an image printed with a decolorable colorant and an image printed with a general non-decolorable colorant. Therefore, when a user tried to reuse a printed paper, it was difficult to determine whether the printed paper could be reused when the images on the paper are decolored.